The most economical additives used for improving the antiknock properties of motor fuels are alkyl lead derivatives, but, in view of the toxicity of lead, their use tends to decrease. Moreover, it is known that the high content of polluting products, such as residual hydrocarbons, carbon monoxide and nitrogen oxides, in the combustion gases of motor fuels, generally requires the use of catalytic mufflers, in order to lower this content of polluting products. These catalytic mufflers contain appropriate catalysts very often including among their components at least one metal from group VIII of the periodical classification of elements, said metal being, usually, platinum or another noble metal of the platinum family. Now, it is known that the lead contained in the motor fuel additives, which is conveyed by the combustion gases, quickly poisons the catalysts of the catalytic mufflers making the latter inefficient for the use to which they are destined. Metal derivatives other than lead derivatives have been tested, for example methylcyclopentadienyl tricarbonyl manganese (MTM), but this product has been shown to be a non-negligible polluting agent.
These various problems have led the refiners to the production of gasolines or gasoline components free of lead-containing antiknock agents (e.g., tetraethyl lead) having, nevertheless, a high octane number.
Up to now, the lead-free gasoline produced in the world has been obtained preferentially from the following techniques:
high severity catalytic reforming of naphtha, PA1 alkylation with isobutane of the olefin-containing C.sub.3 -C.sub.4 cuts from catalytic cracking, thermal cracking, coking, visbreaking and steam cracking. PA1 its boiling point corresponds to that of the gasoline components having the lower antiknock properties; PA1 its vapor pressure is not a disadvantage, PA1 it has an excellent freezing point, PA1 its solubility in water is relatively low and, as it is completely miscible (with) all the hydrocarbons, it has but little liability of causing problems of phase separation in motor fuel mixtures, even in the presence of water. PA1 U.S. Pat. No. 2,417,647 wherein the catalyst contains fluorinated alumina whose hydrofluoric acid content is from about 0.5 to 10%. PA1 U.S. Pat. No. 3,558,733 wherein the alumina-containing catalyst is reactivated with steam without any contact between steam and the olefins to be isomerized. PA1 U.S. Pat. No. 3,558,734 wherein the halogenated catalyst contains about 5 to 100 ppm of water. PA1 U.S. Pat. No. 2,422,884 wherein the operation is conducted in the presence of steam (molar ratio H.sub.2 O/HC varying from 0 to 10) with a boron-containing alumina catalyst. PA1 7.2 cc of pure ethyl silicate PA1 2 cc of a palladium nitrate aqueous solution containing 0.1% of Pd and PA1 60 cc of absolute ethyl alcohol. PA1 3% of SiO.sub.2 PA1 20 ppm of palladiumm and the balance to 100% of alumina.
Lead-free gasoline obtained by high severity catalytic reforming is not ideal as far as pollution and public health are concerned. As a matter of fact, it contains benzene whose vapor is very toxic.
On the contrary, by alkylation, there is obtained a gasoline which is satisfactory as well from an ecological point of view as from a purely technical point of view for the engine operation.
Unhappily this method is essentially limited by a lack of isobutane.
As a matter of fact, the reaction between isobutane and a C.sub.3 or C.sub.4 -olefin is equimolecular; by calculation, it is established that theoretically 1.38 kg of isobutane is required for 1 kg of propylene or 1.035 kg of isobutane for 1 kg of butenes.
However, the olefinic cuts, produced for example by steam-cracking and catalytic cracking, suffer, as a general rule, from a heavy insufficiency of isobutane to satisfy the above-mentioned stoichiometry. For example, a typical cut issued from catalytic cracking has the following composition in percent by weight:
______________________________________ propene 25.00 propane 8.35 isobutane 23.35 isobutene 10.65 n-1-butene 6.65 n-2-butene 18.00 n C.sub.4 (n-butane) 8.00 ______________________________________
A simple calculation shows that the isobutane proportion is hardly one third of the stoichiometrical proportion of olefins.
The problem of isobutane insufficiency of C.sub.3 -C.sub.4 cuts is well known. For example the U.S. Pat. No. 3,758,628 proposes to cope therewith by juxtaposition of a hydrocracking unit to a catalytic cracking unit. But at the present time, it is observed that the number and the capacity of the existing hydrocracking units are either stagnant or even reduced. Moreover hydrocracking is a costly operation which provides a number of products other than isobutane, which cannot always be upgraded.
It will be shown that the present process, according to the invention, provides for a beneficial use of C.sub.3 -C.sub.4 cuts formed of the effluents of catalytic cracking or steam-cracking or coking or thermal cracking or visbreaking units.
As a matter of fact, attempts have been made these last years, to incorporate alcohols, esters, etc. in gasoline either for improving the octane number or to cope with a shortage of petroleum products, or for other purposes. Such attempts are disclosed, for example, in U.S. Pat. No. 3,726,942 and French Pat. No. 2,063,939.
Thus, methanol, which improves the octane number of gasoline, is one of the most interesting additives.
Another interesting additive is methyl-tert-butyl ether (MTBE) whose antiknock properties permit improvement of the quality of gasolines in the trade, in that its addition results in higher octane number than that obtained by use of methanol. Moreover, methyl-tert-butyl ether (MTBE) has a higher calorific value than methanol: 8,935 kcal/kg (i.e. 4.18.times.8,395 kJoules/kg) for MTBE as compared with 4764 kcal/kg (i.e. 4764.times.4.18 kJoules/kg) for methanol (as an average, the calorific value of a premium gasoline is 10,200 kcal/kg corresponding to 4.18.times.10,200 kJ/kg). Moreover, MTBE does not rise demixion problems in the presence of water, as it is the case for methanol. Furthermore, the solubility in water of MTBE is substantially higher than that of water in hydrocarbons and, accordingly, the addition of MTBE improves the compatibility with water of motor fuels.
MTBE has other advantages:
Briefly stated, MTBE appears as a very interesting additive for improving the qualities of gasoline. This product is generally obtained from isobutene and methanol according to the following balanced reaction: ##STR1##
A source of isobutene is the isobutene contained in C.sub.3 -C.sub.4 olefinic cuts issued from effluents of catalytic cracking, steam-cracking, coking, visbreaking and thermal cracking (another source of isobutene is the fraction issued from an effluent of a MTBE producing unit, this fraction being obtained after treatment of said effluent so as to remove therefrom the produced MTBE, optionally withdrawing the paraffins contained therein and optionally hydrogenating the butadiene).
However, it must be observed that a production of 100 000 T/year of MTBE from, for example, a C.sub.3 -C.sub.4 steam-cracking cut, requires a cracking unit producing 500 000 T/year of ethylene, and that in the olefinic C.sub.3 -C.sub.4 cuts issued from catalytic cracking the isobutene contents are still lower than those obtained from a steam-cracking effluent.
The shortage of isobutene is thus incompatible with the extent of development of the MTBE production.
A new way of producing isobutene has thus been proposed. Instead of separating the isobutene present in the above-mentioned C.sub.3 -C.sub.4 cuts, this method modifies the composition of the C.sub.4 hydrocarbons of the olefinic C.sub.3 -C.sub.4 cuts, by isomerizing completely or almost completely the butenes contained in the C.sub.3 -C.sub.4 cuts to isobutene.
Many processes and many catalysts have been proposed for this purposes and particularly catalysts containing aluminas, more particularly activated aluminas (e.g. eta alumina and gamma alumina), halogenated aluminas, bauxite, aluminas treated with derivative or boron or silicium or zirconium, various silicas-aluminas, more or less complex phosphates, solid phosphoric acid, etc.
The process according to the invention is an improvement of the prior art and, particularly, of that disclosed in the following patents:
U.S. Pat. No. 2,395,274, wherein the catalyst is bauxite and the operation is conducted in the presence of steam.
The results obtained with these various processes are still insufficient.
Thus, the catalysts containing fluorinated alumina are not always, in the prior art, the best isomerization catalyst. In spite of a very good selectivity, they have the disadvantage, in view of their acidic properties, to activate not only the isomerization reaction but also undesirable polymerization and cracking secondary reactions which result in a lower selectivity and in a decrease of the catalyst life time. Such catalysts furthermore require a continuous introduction of fluorine to compensate for the losses of fluorine during the reaction and these catalysts also suffer from the corrosive action inherent to the fluorine derivatives.
The catalysts presently used suffer from drawbacks, the most important of which are a low selectivity, resulting from the occurrence of parasitic reactions such as cracking and polymerization, and a lack of stability resulting in a more or less rapid decrease of the conversion rate; in addition, these catalysts are difficult to regenerate or even not regenerable at all.
Furthermore, in order to obtain, with these catalysts, sufficient conversion rates the space velocities must be low enough, thus requiring the use of reactors of a large capacity.